1. Field of the Invention
This invention relates to a plasma treatment method and a plasma treatment apparatus which are used when material gases are decomposed by utilizing the phenomenon of discharge, to form deposited films on substrates or to etch or surface-modify the deposited films formed on substrates. More particularly, this invention relates to a plasma treatment method and a plasma treatment apparatus which are to form on substrates deposited films, in particular, functional deposited films, especially amorphous semiconductors used in semiconductor devices, electrophotographic light-receiving members, image input line sensors, imaging devices, photovoltaic devices and so forth.
2. Related Background Art
As device members used in semiconductor devices, electrophotographic light-receiving members, image input line sensors, imaging devices, photovoltaic devices and other various electronic devices and optical devices, non-single-crystal deposited films such as amorphous silicon as exemplified by amorphous silicon compensated with hydrogen and/or halogen (e.g., fluorine or chlorine) or crystalline deposited films such as diamond thin films have been proposed, some of which have been put into practical use. Such deposited films are formed by plasma CVD (chemical vapor deposition), i.e., a process in which material gases are decomposed by glow discharge produced by high-frequency or microwave power, to form deposited films on substrates made of stainless steel, aluminum or the like. Treatment methods and treatment apparatus therefor are also proposed in variety.
As an example of such apparatus, FIGS. 12A and 12B diagrammatically illustrates an example of the construction of a conventional apparatus for producing electrophotographic light-receiving members by high-frequency plasma CVD. FIG. 12A is its vertical cross-sectional view, and FIG. 12B, a transverse cross-sectional view along the line 12B-12B in FIG. 12A.
This apparatus is constituted basically of a deposition system 1001 having a reactor 1004 formed of a cylindrical dielectric member, a feed system 1002 for feeding material gases into the reactor 1004, and an evacuation system 1030 for evacuating the inside of the reactor 1004.
The deposition system 1001 has a first space 1005 formed inside the reactor 1004 and a second space 1006 formed between the reactor 1004 and a shield wall 1017. Cylindrical substrates 1010, members on which deposited films are formed, are each set to a substrate holder 1012 and is placed in the first space 1005. Also, in the first space 1005, a heater 1016 for heating each substrate from its interior and a material gas feed pipe 1015 are provided. Meanwhile, in the second space 1006, cathode rodlike electrodes 1011 are provided in substantially parallel to the sidewall of the reactor 1004, and a high-frequency power source 1040 is connected thereto via a high-frequency matching device 1041. The material gas feed system 1002 has cylinders (not shown) individually holding therein material gases such as SiH4, GeH4, H2, CH4, B2H6 and PH3, valves (not shown) and mass flow controllers (not shown). The individual material gas cylinders are connected to the material gas feed pipe 1015 leading to the inside of the reactor 1004 via a valve 1026.
Using such a deposited film formation apparatus, deposited films are formed on the cylindrical substrates 1010 in the following way, for example.
First, the cylindrical substrates 1010, having been precisely cleaned in a dust-controlled environment such as a clean room, are each set to the substrate holder 1012 and disposed in the reactor 1004. Then, the inside of the reactor 1004 is evacuated by means of the evacuation system 1030.
Subsequently, a substrate-heating gas for heating the cylindrical substrates 1010 is fed into the reactor 1004 via the material gas feed pipe 1015. Next, by means of a mass flow controller (not shown), the substrate-heating gas is regulated so as to flow at a prescribed flow rate. To do so, the extent of opening of an evacuation valve 1031 is so regulated, watching a vacuum gauge (not shown), that the internal pressure of the reactor 1004 may come to be a prescribed pressure of, e.g., 133 Pa or below. At the time the internal pressure of the reactor 1004 has become stable, the temperature of each cylindrical substrate 1010 is controlled by the substrate heater 1016 to a prescribed temperature of from 50° C. to 450° C.
At the time the cylindrical substrates 1010 have come to have a prescribed temperature, material gases are fed into the reactor 1004 regulating each material gases so as to flow at a prescribed flow rate by means of mass flow controllers (not shown). To do so, the extent of opening of the evacuation valve 1031 is so regulated, watching a vacuum gauge (not shown), that the internal pressure of the reactor 1004 may come to be a prescribed pressure of, e.g., 133 Pa or below.
At the time the internal pressure of the reactor 1004 has become stable, the high-frequency power source 1040 having a frequency of, e.g., 105 MHz is set at a prescribed power and the high-frequency power is supplied into the reactor 1004 through the high-frequency matching device 1041 to cause glow discharge to take place. By the energy of this discharge, the material gases fed into the reactor 1004 are decomposed, so that the desired deposited films composed chiefly of silicon are formed on the cylindrical substrates 1010.
After the deposited films have come to have the desired layer thickness, the supply of high-frequency power and flowing of material gases into the reactor 1004 are stopped to finish the formation of deposited films.
Then, the like procedure may be repeated a plurality of times to form light-receiving layers having the desired multi-layer structure.
Here, needless to say, valves other than those for necessary gases are closed when respective layers are formed. Also, the operation to full open the evacuation valve 1031 to once evacuate the inside of the system to a high vacuum is optionally made in order to avoid the respective gases from remaining in the reactor 1004 and in the piping which leads to the reactor 1004. Also, during the formation of deposited films, the cylindrical substrates 1010 are rotated by driving a motor 1020.
In the case where plasma treatment is made in this way, the impedance on the load side and the impedance on the high-frequency power source side are matched by means of the high-frequency matching device 1041. The impedance on the load side involves a stray capacitance component, an inductance component and a resistance component, and hence may greatly change depending on the conditions for plasma treatment and the shape of the apparatus for making the plasma treatment. Hence, the regulation of impedance requires specific values for each apparatus or for each plasma treatment condition.
As a method for matching impedances, it is common to match impedances by changing the capacitance of variable capacitors in a π-type or T-type circuit provided in the matching device. Also, when it is insufficient to regulate the impedance only in the matching device, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 9-310181, capacitors are attached individually to a plurality of cathode electrodes so that the distance between the matching device and the cathode electrodes can be made larger whereby any changes in the induction component can be cancelled to match impedances. As also disclosed in Japanese Patent Application Laid-Open No. 8-253862, the length of an electrode lead-in shaft connected to a plasma-generating electrode and that of a coaxial cylindrical earth shield are set variable so as to enable adaptation to a variety of power source frequencies.
Such conventional methods and apparatus have attained a good state of matching. However, there is further room for improvement when it is intended to form deposited films in a good efficiency in actual production.
The above method for matching can certainly attain a good matching in respect of certain plasma treatment. When, however, the electrophotographic light-receiving members described above are produced, electrophotographic light-receiving members different in shape and film composition must be produced in conformity with electrophotographic apparatus greatly rich in variety. Accordingly, the impedance of reactors for forming deposited films changes. Moreover, in the case of multi-layer construction like the electrophotographic light-receiving members, the type of treating gas, the internal pressure, the high-frequency power and so forth change for each layer, and hence the impedance ascribable to plasma may also greatly change.
As a result, in conventional plasma treatment systems, an attempt to well match impedances in accordance with various forms of products may make it necessary to provide matching devices specifically designed for respective conditions, resulting in a high cost for the whole apparatus and furthermore providing an obstacle to the cost reduction of articles to be produced. Also, the matching device must be replaced every time the conditions for plasma treatment have changed. This causes a lowering of operating efficiency. Also, when any treatment under the like conditions is continuously made in order to prevent the operating efficiency from lowering, the flexibility of production may be held back, making it difficult to smoothly execute the adjustment of production that may have to be made because of a variety of production requirements or any accidental troubles.
Accordingly, in plasma treatment systems making use of high-frequency power as stated above, it has been sought to simplify production systems against manufacture of many kinds of articles, and to construct a plasma treatment apparatus, or early materialize a plasma treatment method, which can achieve low cost.